As we see birds flying overhead this fall, I want to share insight on bird migration from Dr. Jim Pease, retired ISU Extension Wildlife Specialist.
Migration is the movement of animals from one place to another. People have been fascinated with this annual migration of birds for thousands of years. Aristotle was an ancient philosopher who wrote about the wintering habits of birds 3,000 years ago. He noticed that some birds traveled to warmer places to spend the winter. He also mistakenly believed that some birds hibernated to survive the harsh winter weather. This theory persisted for 2,000 years!
Today, we know that birds do not hibernate. But it does show how long people have been trying to understand the disappearance of many birds from northern climates in the fall.
When you see flocks of birds flying overhead in the fall, they usually are flying south to their wintering grounds. How far south they go depends on the species of bird.
The largest group of birds that we see during migrations are called "neotropical" migrants. They got this name because these species of birds migrate in the fall all the way to Mexico, the Caribbean islands, and other Central American and South American countries in the tropics. This means these birds fly thousands of miles every fall and spring. About 300 of the 650 bird species that nest in North America are neotropical migrants. They include warblers, orioles, hummingbirds, swallows, swifts, shorebirds, and some birds of prey. The neotropical migrants make up 50-70 percent of the bird species of deciduous forests and prairies in the central United States.
Many birds cross open ocean during their migration between North and South America. This means that birds need a lot of energy to migrate. This energy is stored in the form of body fat. Smaller birds cannot store a lot of fat to use as energy during long flights. During migration, some birds lose as much as one fourth to one half of their entire body weight, so it is very important that they store up enough fat for energy. How smaller birds ever store enough to make these flights is still a wonder to scientists. It was once believed that little birds, like hummingbirds, migrated by riding on the backs of larger birds. However, this myth is not true. These little birds make it entirely on their own!
Some birds need to stop to rest and feed during the day. This is when insects they eat are most active and available. These birds, then, migrate at night. They can find their way at night because they learn to follow the rotation of the stars. On cloudy nights, wind direction may help them to orient themselves. Other birds, like barn swallows, migrate during the day and feed on flying insects while they are in the air. That way, they aren't limited to traveling at night because they feed during flight.
When birds migrate is closely tied to why they migrate in the first place. Primarily, birds go south for the winter to find lots of insects and other food. However, these birds need more room and even more insects during the breeding season when they have a nest of young ones to feed. To solve this problem, the birds migrate north for the summer. As a result, when their bodies become ready to breed every spring birds know it is time to migrate north. In the fall, their body puts on fat and signals that it is time to begin their long journey south. Actual migration begins when the birds are triggered by some other stimulus, such as a change in temperature or weather.
As you can see, bird migration is a very complex behavior that has evolved over many years. If you have any questions, check out this link to our Extension Wildlife pages of our website http://www.extension.iastate.edu/wildlif.... This section is full of interesting and helpful information about the wildlife you live with everyday